It is well known that an R-T-B based permanent magnet (wherein R represents a rare earth element, T represents Fe or Fe with part of it substituted by Co) with a tetragonal compound R2T14B as its main phase has excellent magnetic properties, and is a representative permanent magnet with high performance since it was invented in 1982 (Patent Document 1: JP S59-46008A).
The R-T-B based magnets with rare earth element(s) R composed of Nd, Pr, Dy, Ho, Tb have a large magnetic anisotropy field Ha and are preferred as a material for a permanent magnet. Among them, the Nd—Fe—B based magnet with Nd as the rare earth element(s) R is widely used, because it has a good balance among the saturation magnetization Is, the Curie temperature Tc and the magnetic anisotropy field Ha, and is better in resource amount and corrosion resistance than R-T-B based permanent magnets using other rare earth elements R.
As a power unit used in people's livelihood, industries and transportation equipments, the permanent magnet synchronous motor has been used. However, the permanent magnet synchronous motor in which the magnetic field generated by the permanent magnet is constant is hard to drive, as the induced voltage increases in proportion to the rotational speed. Thus, when the permanent magnet synchronous motor is operated in an intermediate/high speed region or under a low load, it is necessary to perform a field-weakening control which cancels out the magnetic flux of the permanent magnet with the magnetic flux generated by the armature current, so that the induced voltage will never exceed the supply voltage. As a result, there is a problem that the efficiency of the motor will decrease.
In order to solve the problem mentioned above, a motor with variable magnetic flux using a magnet in which the magnetic force changes reversibly when an external magnetic field is applied (a magnet with variable magnetic force) is developed. When the motor with variable magnetic flux is operated in the intermediate/high speed region or under a low load, the decrease of efficiency in the motor due to the field-weakening effect in the prior art can be inhibited by reducing the magnetic force of the magnet with variable magnetic force.